The present invention relates to a paper machine suction roll comprising a revolving mantle loop and a stationary suction shoe arranged inside the mantle loop and connected to a source of negative pressure.
The present invention also relates to a method for dewatering a web in a forming section having a twin-wire forming zone in which the web passes over at least one suction roll comprising a revolving mantle loop and a stationary suction shoe.
Suction rolls are typically employed at the wet end in paper machines, i.e., in connection with the wire part and the press section, for example, as a web forming roll, couch roll, pick-up roll, felt conditioning roll, and press roll.
Prior art suction rolls typically consist of a revolving perforated mantle cylinder and an axial suction box placed inside the cylinder. The suction box is arranged to follow an inner face of the cylinder mantle by means of seal ribs. The width of the suction zone in a typical suction box is usually from about 100 mm to about 500 mm and the suction box extends from end to end in the mantle. The suction box communicates with a suction system so that negative pressure is produced. Air flows through holes placed in the sector in the cylinder mantle of the suction roll which faces the suction box at each particular time during rotation of the roll.
Prior art suction rolls operate in a manner so that the wet paper web formed in the former of a paper machine is passed on support of a wire or felt over the suction zone of the suction roll. The negative pressure effective at locations within the suction roll promotes the removal of the water, which is separated from the web, and its flow into the structure of the wire or the felt and, further, into the holes in the suction roll. By the effect of the suction, water may enter through the holes into the suction box, or water may also remain in the holes in the suction roll. In the latter case, the water remains in the holes as long as the holes are subjected to the effect of the suction and air flows through the holes. However, the water is ejected out of the roll after the holes have passed beyond the suction zone.
The thickness of the cylinder mantles of prior art suction rolls is typically from about 30 mm to about 100 mm depending on the other dimensions of the roll. The roll diameter and the mantle thickness are selected so that the deflection of the suction roll remains within permitted limits during operation of the paper machine.
Generally, a suction roll situated in a wire part has from about 10,000 to about 12,000 holes per m.sup.2, and the diameter of each hole is from about 5 mm to about 6 mm. In the suction rolls arranged in the press section, the number of holes is higher, but the diameter of each hole is smaller, e.g., from about 4 mm to about 5 mm.
Suction rolls are considered expensive parts of paper machines in relation to the other individual components of the paper machine. In particular, the drilling of a large number of holes into the roll produces high manufacturing and related costs. The perforations, i.e., holes, reduce the strength of the mantle, for which reason it is necessary to use special metal alloys as the raw material of the rolls as well as a relatively thick mantle. Thus, there is also a high cost of material to produce the suction rolls which results in high manufacturing cost.
The quantity of air that enters into the suction box in a suction roll and that must be dealt with by the suction pump in the suction system communicating with the suction rolls is derived from three sources:
1) from the air coming through the web, PA0 2) from the air entering into the suction zone along with the holes during each revolution of the suction roll ("hole air"), and PA0 3) from stealth air, which enters into the suction box as a result of seal leaks. The amount of stealth air is usually quite low as compared with the other two air quantities.
The following example gives an idea of the ratio between the first two afore-mentioned quantities of air, i.e., the air coming through the web and the "hole air". The numbers provided below refer to the characteristics of a paper-machine suction roll whose length is about 10 meters and in which the width of the suction box is about 110 mm, and the negative pressure applied to produce the suction effect is about 65 kPa. At a machine speed of about 1500 meters per minute, the proportion of air coming along with the holes is about 260 m.sup.3 per minute, and the proportion of the air passing through the web is less than about 200 m.sup.3 per minute.
With modern high-speed paper machines, the amount of air that enters into the suction zone of the suction roll, and suction system connected thereto, along with the air passing through the holes, i.e., the hole air, has proved to be surprisingly high. As the running speeds of paper machines increase, the proportion of the "hole air" will also increase. This proportion is increased further by the fact that, with increasing machine speeds, the rolls must be made ever stronger. Rolls are made stronger by, e.g., increasing the thickness of the mantle. Thus, since the amount of hole air is proportional to the thickness of the roll mantle, an increase in the thickness will have a corresponding increase in the amount of "hole air" inpassed into the suction system.
In order to reduce related utility expenses of operating the suction system to compensate for the "hole air", it is desirable to reduce the proportion of "hole air" to a practically insignificant level. For example, in a newsprint machine whose running speed is about 1500 meters per minute and trimmed width about 9.5 meters, the total suction pump capacity required for dealing with the hole air, with respect to all the suction rolls in the newsprint machine, is about 72,000 m.sup.3 per hour, and the corresponding motor power connected to the suction pumps is about 1600 kW. If the suction pump capacity can be lowered by about 1000 kW, this results in a savings of more than about 7 million kWh per year. Therefore, there is a considerable advantage to reducing the amount of hole air passed into the suction system.
A particular operational and technical drawback related to prior art suction rolls used in paper machines is that the suction rolls produce intensive noise which can cause even serious damage to the health of the workers operating the paper machine. This noise is generated since the holes in the suction roll operate as a sort of whistles. When the holes under vacuum enter outside the suction zone, they are filled with air as a pulse which produces a strong whistling sound having a basic frequency determined by the length of the drill pattern of the holes. The system of whistles formed by the high number of holes in the suction roll often produces a noise that exceeds the pain threshold of the ears.
In the prior art, attempts have been made to attenuate this noise by means of various arrangements, for example, by using a suitable drilling pattern of the holes or sound-insulating walls. However, in practice it has not been possible to achieve significant attenuation of this noise by means of the prior art solutions. It is thus desirable to significantly reduce the noise of the suction rolls to inhibit the related problems.
With respect to the prior art related to the present invention, reference is made to published Finnish Patent Application No. 762620 (Matti Kankaanpaa) of the assignee, Valmet Paper Machinery Inc., and to corresponding U.S. Pat. No. 4,172,759, the specification of which is incorporated by reference herein. In this reference, a method is described for subjecting a web, or a fibrous suspension layer, that is passed on support of a felt or wire over a roll in a paper machine, or an equivalent web, wire or felt, to a suction effect. In the method, the sector of the roll which is not covered by the object subject to the suction effect, communicates with the suction system from outside the roll.
Further, this reference describes a roll device that comprises a revolving suction roll provided with through perforations, or a corresponding grooved solid-mantle roll, and a suction chamber extending over a considerably large sector of the roll. The suction chamber is provided with a mantle whose edges have seal parts placed in contact with the roll. The ends of the suction chamber have seals that are in contact with the outer faces of the ends of the roll mantle. The roll device also includes members arranged to facilitate the connection between the suction chamber and a suction pump and additional members arranged to remove the water collected in the interior of the suction chamber.
In prior art suction rolls, it is a further drawback that, in some positions, the suction roll tends to apply a marking to the paper web corresponding to the hole pattern in its mantle.
Also, in the prior art, suction devices placed in the wire part of a paper machine are known to include a perforated belt fitted between two guide rolls. The belt has a straight planar run between the guide rolls which is fitted against the inner face of the forming wire. A suction box is arranged inside the belt loop. These devices have not been used more commonly because one of their drawbacks are problems related to the structure and the control of the perforated belt, including transverse instability.
The highest running speeds of paper machines currently in operation are already in a range of about 1500 meters per minute, and at present, machines are being contemplated whose speeds will be in the range of at least about 2000 meters per minute. With these high running speeds, the problems discussed above will be manifested with increased emphasis. With increasing running speeds and widths of paper machines, it is also necessary to increase the diameters of the suction rolls. However, the raw-materials and technical aspects of the roll production processes, in particular centrifugal casting of the roll mantle, impose limitations on the construction of large-diameter suction rolls.